chromium/third_party/blink/web_tests/external/wpt/html/editing/dnd/draggable-areas/z-index.html

<!DOCTYPe html>
<meta charset='utf-8'>
<title>drag and drop – feedback overlay for heavily styled elements – 008</title>
<style>
a {
  display: block;
  height: 200px;
  width: 200px;
  background-color: blue;
  position: absolute;
  top: 100px;
  left: 10px;
  z-index: 1;
}

div {
  background-color: orange;
  position: absolute;
  height: 200px;
  width: 200px;
  top: 150px;
  left: 20px;
  z-index: 2;
  opacity: 0.9;
}


</style>

<p>Click and hold the part of the orange box that overlaps the blue box. Then
move your pointing device. The blue box should <em>not</em> be dragged.

<a draggable="true" ondragstart="event.dataTransfer.effectAllowed ='copy'">TEST</a>

<div></div>